In a study on an organic electroluminescent (EL) device (hereinafter, referred to as ‘organic EL device’), which has continued from an organic thin film light emission observation by Bernanose in 1950 to blue electroluminescence using an anthracene single crystal in 1965, an organic EL device having a lamination structure including functional layers of a hole layer and a light emitting layer was proposed by Tang in 1987. Subsequently, the organic EL device has been developed in a form of introduction of a specific organic layer into a device and a specific material used therein has been developed in order to manufacture the organic EL device having high efficiency and long lifetime.
The organic EL device is formed of an ITO (indium tin oxide) substrate, an anode, a hole injection layer selectively receiving holes from an anode, a hole transport layer selectively transporting the holes, a light emitting layer emitting light by re-bonding the holes and the electrons, an electron transport layer selectively transporting the electrons, an electron injection layer selectively receiving the electrons from a cathode, and the cathode. As described above, the reason why the organic EL device is manufactured to have multilayers is because moving speeds of the holes and the electrons are different from each other, and if the appropriate hole injection layer, hole transport layer, electron transport layer, and electron injection layer are manufactured, the holes and the electrons can be efficiently transported, and a balance of the holes and the electrons in the device may be ensured, thus increasing light emitting efficiency. The electrons injected into the electron injection layer and the holes transported to the hole injection layer are re-bonded in the light emitting layer to form an exciton, light emission by falling from a singlet excited state to a bottom state is called fluorescence and light emission by falling from a triplet excited state to a bottom state is called phosphorescence. Theoretically, when carriers are re-bonded in the light emitting layer to generate the exciton, a ratio of the singlet exciton and the triplet exciton may become 1:3, and in the case where phosphorescence is used, internal quantum efficiency may be 100%.
Meanwhile, generally, a carbazole-based compound such as CBP (4,4-dicarbazolybiphenyl) is used as a phosphorescent host material, and a metal complex compound including heavy atoms such as Ir and Pt is extensively used as a phosphorescent dopant material. However, in the case of CBP that is the currently used phosphorescent host material, there are problems in that a glass transition temperature (Tg) is about 110° C., which is low, and lifetime of the organic EL device is about 150 hours, which is very short, because crystallization easily occurs in the device.